The Discovery of a P Cygni Analog in M31

Abstract

We present spectroscopy and discuss the photometric history of a previously obscure star in M31. The spectrum of the star is an extremely close match to that of P Cygni, one of the archetypes of luminous blue variables (LBVs). The star has not shown much variability over the past 40 years (<0.2 mag), although small-scale (0.05 mag) variations over a year appear to be real. Nevertheless, the presence of a subarcsecond extension around the star is indicative of a past outburst, and from the nebula's size (0.5 pc diameter) we estimate that the outburst took place roughly 2000 years ago. P Cygni itself exhibits a similar photometric behavior and has a similar nebula (0.2 pc diameter). We argue that this may be more typical behavior for LBVs than commonly assumed. The star's location in the H-R diagram offers substantial support for stellar evolutionary models that include the effects of rotation, as the star is just at a juncture in the evolutionary track of a 85 M_solar star. The star is likely in a transition from an O star to a late-type WN Wolf-Rayet star.

Based in part on observations made with the NASA/ESA Hubble Space Telescope (HST), obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association for Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555.